Mobile and/or portable radio transceiving devices have heretofore incorporated a "channel guard" decoding option which permits an operator to selectively call desired parties by transmitting a low frequency tone or digital data pattern. The "channel guard" coding option (which is also referred to as a continuous tone coded squelch system (CTCSS) or a continuous digital coded squelch system (CDCSS)) provides a means of restricting calls to specific radios. Only the desired parties' receivers are programmed to decode the transmitted tone or digital data pattern.
By using the channel guard option, many users can share a repeater system with only the receivers programmed to receive the particular transmitted channel guard code being enabled to receive a transmitted message. The transmitted tones in a tone channel guard system may, for example, range from 67 Hz to 210.7 Hz. in 0.1 Hz steps.
Radio systems incorporating tone channel guard features include tone processing circuitry for processing and decoding the received tones to detect a proper tone sequence. To process such low frequency tones to correctly detect predetermined channel guard tone patterns, it is necessary for the tone processing circuitry to include RC time constants which are long enough to preserve the tone signal pattern to permit reliable decoding of noisy signals. Thus, a considerable period of time, e.g., 250 milliseconds for 100 Hz, may be required to decode a channel guard pattern.
Mobile and/or portable radio transceiving devices have also heretofore incorporated channel scanning to permit monitoring a priority channel while listening to an active channel. A priority channel is a channel which is designated to be the channel on which the radio operator can always be reached. Such a radio's channel scanning algorithm controls the scanning of the activity on the channels to check for activity on the priority channel more frequently than activity on any other channel. If activity is detected on the priority channel, the radio immediately switches to the priority channel.
In mobile radio communication systems utilized by public services (such as the state police, highway patrol, or fire department), there is a need and an expectation that, as soon as a message is transmitted on a priority channel, the receiving party will be permitted to listen to the message. For example, a dispatcher communicating the location and important details of an ongoing robbery or fire, demands a rapid return to the priority channel without any significant delay.
Mobile radio communication systems utilized by such public service organizations have heretofore included channel guard in their scanning algorithms to take advantage of the ability to share a repeater system with only the receivers programmed to receive the particular transmitted channel guard being enabled to receive the transmitted message. However, such public service mobile radio communication systems, have not heretofore included channel guard for the priority channel.
In this regard, in such conventional radios, it may take up to 400 milliseconds to detect the carrier and channel guard tone pattern. Such a time period is much too long for the typically short transmission made during public service operations. Thus, checking a priority channel for carrier activity and decoding a channel guard tone pattern will place a hole in the active channel's audio which may serve to delete words from a conversation. Clearly, the deletion of words from an emergency communication cannot be tolerated. Thus, existing radios which incorporated scanning for channel guard, but which stopped on a channel for a fixed interval of 400 milliseconds (and which therefore left significant gaps in the audio) are not satisfactory for use by state police, the highway patrol or other public service organizations.
In a product designed for business and industrial use, General Electric's MLS radio system permitted tone channel guard decode and scan to coexist on the priority channel by incorporating a "partial look" technique for determining whether a channel guard tone pattern might be present. If this initial look indicates that the channel guard pattern is not present, then the unit returns to the non-priority channel. If, however, the initial look indicates that the channel guard might be present, then the unit stays for an additional time interval window to guarantee the correct channel guard is present before opening the audio.
This approach, while cutting down on the length of time required to spend on decoding of the channel guard on the priority channel and allowing more time to listen to the correct audio on the non-priority channel, was still not satisfactory to meet public service organization needs. In this regard, the partial look required on the order of 175 milliseconds to determine whether channel guard might be present. In this system, a fixed time interval is utilized for all the channel guard patterns. Additionally, in this system, the audio is not unmuted until the correct channel guard is decoded. Accordingly, in the MLS radio system, significant portions of an emergency communications on a priority channel would be lost.
The present invention incorporates a significantly improved scanning algorithm which allows tone channel guard decode and scan to optimally coexist on the priority channel. In an exemplary embodiment of the present invention, the scanning algorithm divides the tone channel guard decoding processing into three processing windows. The first window (hereinafter referred to as SHORTLOOK) serves to indicate whether channel guard might be present.
Instead of employing a fixed time period for all channel guard frequencies, the present invention uses a method of scaling the timing windows. For example, the SHORTLOOK time period is set depending on the channel guard frequency being decoded so that when decoding high frequency tones (e.g., 210.7 Hz), a determination may be made to abort the decoding operation and return to the active channel with a smaller window which results in a minimized hole in the audio. A timer is utilized to define the variable length of the SHORTLOOK window. The SHORTLOOK window is set to such a short period of time that the tone cannot be guaranteed to be present but it is long enough to determine that a tone is not present.
During the SHORTLOOK window, an input channel guard tone pattern is repetitively sampled during a predetermined number of interrupt periods. Depending upon the state of the sampled tone pattern at each interrupt, a value is associated with the sample which accumulates over time (and which is hereinafter referred to as the vector of the sampled tone pattern). The growth of this vector over time indicates the likelihood of the presence of the correct channel guard tone pattern. At the end of the SHORTLOOK time period, if the magnitude of the vector exceeds a predetermined threshold, then a flag is set indicating that a correct channel guard tone pattern might be present.
If the SHORTLOOK window processing indicates that the correct channel guard might be present, then a further processing window, hereinafter referred to as SECONDLOOK, is initiated. If at any time during the SECONDLOOK time period, a second predetermined threshold is exceeded, the system is informed that the correct channel guard tone pattern is most probably present. The audio is then unmuted, even though the SECONDLOOK time period expires before the channel guard tone pattern is capable of being reliably detected in accordance with conventional standards.
If the first or second thresholds are not exceeded during the SHORTLOOK or SECONDLOOK windows, then the scan returns to the non-priority channel. Thus, the present invention serves to shorten the time period in which holes appear in the active channel audio. If, however, during the SECONDLOOK window, the second threshold is exceeded, then the priority channel audio is opened so that for, example, a police officer may receive communications prior to the time period that is conventionally required to perform a full length tone channel guard decode operation. By utilizing the SECONDLOOK window, the time period of the SHORTLOOK window may be minimized Thus, a determination that a tone channel guard is not present may be quickly made to keep the audio blanking time down to the shortest possible time.
In accordance with the present invention, a LASTLOOK window is utilized when the SECONDLOOK window has detected a tone and opens the audio. During the LASTLOOK window, a further tone channel guard detect threshold is monitored. If this threshold is exceeded, then the priority channel audio is allowed to remain active, thereby serving as a check on the determinations made in the SHORTLOOK and SECONDLOOK windows.
The present invention uses a plurality of scan rates to minimize the percentage of time audio is lost while looking at the priority channel and listening to a non-priority channel. If no activity is detected on the priority channel, then the priority channel is sampled rapidly, e.g., every 300 milliseconds. If, however, there is carrier activity on the priority channel, but an incorrect tone channel guard, then the priority channel is sampled at a slower rate, e.g., every second. This results in a constant blanking time to minimize lost audio on the active non-priority channel.